I have 2 questions regarding betta fish firstly how much rock salt is to be added per gallon of water ?cus on the web it ranges from 1 teaspoon per gallon to 1 teaspoon per 5 gallons
my second question is can someone give me a detailed explanation about velvet or any links if possible?

Why are you wanting to use salt? For daily use, you do not want to use salt with your freshwater betta as it can cause irreversible and deadly health problems if over exposed - and for treatment it depends on what you are treating.

First off what is the salt for? As an additive or to actually medicate? Surprisingly, velvet is one thing I have never actually had to deal with. I do know people mention to get copper safe, or such medications since they work the best.

Never ever ever ever. Quantity: 0%. The betta fish has a labryinth organ, which with the use of salt long term can cause problems and lead to death. To some species of fish it is beneficial however with surface breathers and those without scales adding salt is a big no no unless medicating for something in which the salt is needed.

As Sena mentioned, salt should only be used when treating an external problem - prolong exposure you are risking their kidneys/liver and the bacterias and parasites that cause fin rot, fungus, ich, velvet, etc will actually become resistant to the salt - therefor it won't keep them away and if they do happen to get one of those things, the salt then won't be effective. You won't be avoiding health problems by using salt, instead you will be creating them.

Best preventative is to keep up with the proper weekly water changes and good food.

There was a girl here whose fish had its fins burned off as a result of her using salt on a daily basis - it wasn't a pretty sight, and I'm not sure if the fish made it or not, so please, take the others' advice, and don't use salt as a disease preventative. The best thing to prevent disease is warm, clean water.

You shouldn't use salt on a regular basis. Bettas are freshwater fish...not saltwater fish. You should only use salt to medicate and even then only in certain situations. Using salt on a regular basis can lead to serious and possibly fatal consequences to your betta.